Fuel bundles for nuclear reactors typically include a plurality of nuclear fuel rods extending in generally parallel relation one to the other and arranged in a rectilinear matrix of fuel rods, e.g., 8.times.8, 9.times.9, 10.times.10 arrays, with peripheral or edge fuel rods surrounding interior fuel rods, as well as one or more interior water rods. Characteristic operating conditions of a BWR fuel bundle lattice are very heterogeneous. For example, large quantities of non-voided water (moderator) lie between the fuel rods adjacent lower portions of the bundle, while boiling water (voided) lies adjacent the upper end of the bundle. This results in reduced average water density. The power in any fuel rod is proportional to the low energy neutron density within the fuel rod. When the neutrons are liberated in the fission process, the neutron energy is transferred to the water within the reactor core through elastic and inelastic collisions, resulting in a shift in the neutron energy spectrum towards the low end, with the highest population of low energy neutrons existing near regions of high water density. It has been observed that in regions of the reactor that contain high water density and exhibit large thermal neutron densities, the fuel rods near those regions exhibit relatively high powers. Particularly, it has been observed that fuel rods about the periphery or edge of the fuel bundle typically operate at powers that are substantially, e.g., on the order of 20%, higher than the majority of the interior rods. Interior fuel rods adjacent one or more water rods also exhibit somewhat elevated powers.
Peak power limit for each fuel rod in a nuclear fuel bundle is defined as the maximum power limit at which each rod may operate, i.e., a maximum power output per unit length of fuel rod during steady state operation. Peak power limit is evidenced by a thermomechanical curve that basically identifies the maximum peak power output at which each rod can operate as a function of time. This limiting curve is the same for all fuel rods in the lattice and all fuel rod positions independent of the number of pellets within the fuel rod, their enrichment, column length, fission gas plenum volume and the like. All rods within the fuel bundle, regardless of type, e.g., fuel rods only, rods having a mixture of fuel with poisons such as gadolinium or part-length fuel rods, must operate below the peak power limit. Because the natural power peaking in a BWR fuel bundle is dependent upon the position of the fuel rod relative to the neutron moderator, i.e., water, it is common to have a subset of fuel rods that dictate the maximum power that is achievable in the fuel bundle. Thus, the fuel rods adjacent the periphery or edge of the fuel bundle typically define the maximum power peaking in the BWR fuel bundle. Stated differently, the interior fuel rods typically operate with a greater margin relative to the peak power limit than do the peripheral or edge rods and, in essence, are under-utilized. To offset that, fissile enrichment in these high-power peripheral fuel rods is often depressed, hence increasing the operating margins for the rods while disadvantageously limiting the power output that can be generated from the fuel bundle.
In known prior designs, all of the fuel rods of a bundle have the same peak power limit and all fuel rods of that bundle operate below the peak power limit with different margins. For example, while a majority of fuel rods in a BWR fuel bundle are uranium rods that do not contain poisons, even those rods which do contain poisons such as gadolinium, as well as part-length fuel rods, must operate below the peak power limit. These rods are typically located within the interior of the bundle. Consequently, when the peak power limit is established and the fuel rods are designed to balance power producing capability and fuel bundle weight, the resulting fuel bundle is fundamentally unbalanced because the fuel rod power behavior is very dependent upon the lattice position of the rods within the bundle such that some rods operate near the peak power limit and others have significant margins.